Intravascular ultrasound (IVUS) catheters provide a means of imaging internal structures in the body. Coronary IVUS catheters are used in the small coronary arteries of the heart to visualize coronary artery disease (CAD). There are other non-IVUS invasive imaging systems and catheters, such as optical coherence tomography OCT and Optical Frequency Domain Imaging (OFDI), that also provide a means of imaging internal structures within the human body. IVUS catheters (see FIG. 1) use transducers 46 to create pressure waves, other imaging systems use different imaging energy sources. Both OCT and OFDI use laser light as the imaging source.
There are two type of coronary IVUS catheters: mechanically scanned and electronically scanned. Mechanically scanned IVUS catheters mechanically spin the ultrasonic beam to sweep across a region of interest in the body. There are two generally accepted ways to do this. One is to mechanically rotate the transducer (FIG. 1) that generates the ultrasound beam. The other is to mechanically rotate a reflective surface or mirror that directs the beam from a stationary transducer into the desired swept pattern.
The preferred method is to rotate the transducer for the following reasons: Mirror based systems have additional imaging artifacts in their images as the beam is swept past mechanical supporting structures. Mirror based systems are generally larger then rotating transducer systems.
Electronically scanned catheters utilize a transducer array that electronically steers the ultrasound beam. In order to maximize the size of the transducer array, electronically scanning IVUS catheters locate the transducer array on the outside of the sheath. Mechanically scanning IVUS catheters locate the transducer on the inside of a sheath.
Mechanically scanning IVUS catheters FIG. 1 have two key advantages over electronically scanning IVUS catheters. Mechanically scanning IVUS catheter transducers 46 can operate at higher frequency then electronically scanning transducers. Therefore they have higher resolution. Mechanically scanning imaging catheters operate within an ultrasonically transparent sheath 28. The sheath prevents rotating components 33 and 34 from coming into contact with the patient's tissue and causing trauma. In addition, the sheath provides a lumen 49 that facilitates the movement of the imaging element along a portion of the distal length of the imaging catheter. Therefore, with a sheathed mechanical scanning IVUS catheter a volume of image data can be acquired over a region of interest without physically moving the catheter sheath 27 and 28 within the body.
Mechanically scanning IVUS imaging catheters contain drive cables 33 to “spin” the transducer 46 within the sheath 26 and 28. Drive cables are currently assembled by winding multiple strands of metal wire on a mandrel to create a long spring containing a repeating series of concentric rings, or windings, of the wire. Two, or more, springs are wound for each drive cable sized one to fit over the other. Adjacent springs are wound in the opposite direction of each other so that the grooves between the windings do not line up and lock together. During assembly, the inner spring is inserted into the outer spring still on its winding mandrel and then released so that it expands into the outer spring. In this way, the drive cable is extremely flexible in order to navigate small tortuous distal coronary anatomy while still providing some degree of torsional rigidity between the proximal driving end and the distal end containing the transducer.
Proximal housing 25 contains engagement pins 38 (×2) that mechanically mate to the imaging system catheter interface port. Within proximal housing 25 is a connector 30 which couples in mechanical energy to the drive cable 33 and electrical energy into the transmission line 47 within the drive cable. Connector 30 is fixedly connected to drive shaft 31, such that when rotated by the imaging system, drive shaft 31 is similarly rotated. Internal drive shaft 31 has a smooth bearing surface 37 which provides the running surface for rotational bearing 36 and snap ring 35. Snap ring 35 is fixedly held in place by the groove in proximal housing 25. A fluid seal 39 prevents fluids from the lumen 49, which runs the length of the catheter, from getting into the connector 30. The distal end of internal drive shaft 31 is connected via solder, brazing, welding or gluing bond joints 32 to the drive cable 33, such that when drive shaft 31 is rotated, drive cable 33 is similarly rotated.
Connector 30 within proximal housing 25 contains an electrical interface to couple in rotating electrical energy into the transmission line 47 that is disposed within drive cable 33 and runs its entire length. Transmission line 47 couples transmit energy from the system via connector 30, through the drive cable 33, and to the transducer 46 located within the distal housing 34. The electrical excitation energy causes transducer 46 to generate a pressure wave into the lumen 49 which is filled with saline via flushing port 40. The ultrasonic energy is coupled via the saline into the ultrasonically transparent portion of the sheath 28 and into the body. Objects in the body having acoustic impedance variations reflect back a portion of the ultrasonic pressure wave which is received by the transducer 46 after passing through sheath 28 and the saline filled lumen 49. Transducer 46 converts the received pressure signals into electrical signals which are coupled via transmission line 47 back to connector 30 and into the imaging systems' receiver. The system converts a series of scan lines acquired in the polar (R, θ) coordinate system, (similar to a beam from a lighthouse) into a slice or frame of image data by converting the polar scan lines into the Cartesian (X,Y) coordinate system for display on a X-Y scanning monitor, thus completing one rotation of the connector 30, drive shaft 31, drive cable 33, and distal housing 34.
In order to move, or translate, the rotating transducer 46 along the distal portion of the length of the lumen 49, a telescopic section is added at the proximal end of the catheter. The telescopic section contains inner proximal tubular element 26, outer distal tubular element 50, and anchor housing 29. The distal end of inner proximal tubular element 26 contains an end stop 51 to prevent the inner proximal tubular element 26 from disengaging from the outer distal tubular member 50 when the telescope is fully extended. Fluid seal 41, inside anchor housing 29 prevents fluids from lumen 49 from leaking out via the space between inner proximal tubular element 26 and outer distal tubular element 50. Groove 52 in anchor housing 29 provides a connection point for motorized (controlled) movement of the distal outer tubular element relative to the proximal housing 25.
Due to the flexible nature of the drive cable 33, the telescope 26, 29, and 50, and sheath 27 and 28 must provide a running surface to support drive cable 33 when it is rotating. It is also important to note that drive cable 33 is of a fixed length, so that when the outer distal tubular element 50 is translated relative to the inner proximal tubular member 26, the transducer 46 is translated relative to the distal sheath 28. In this way, the transducer 46 is moved along the length of the sheath 28 to acquire a volume of image data.
Current telescopic devices on IVUS catheters have several shortcomings. The current telescopes design is based on a proximal inner tubular member 26 that has an inside diameter sized to accommodate the drive cable 33 and provide sufficient clearance for flushing fluid. Flushing fluid is injected into flushing port 40 to fill lumen 49, in order to couple ultrasound energy from transducer 46, through the fluid to sheath 28 and thereby into the patient's body. The wall thickness and therefore the outer diameter of proximal inner tubular element 26 is sized in order to provide adequate structural integrity to support the forces occurring during the movement of the telescopic section in order to reduce the likelihood of any kinking or collapsing of the inner lumen onto the spinning drive cable 33. If the inner lumen of inner proximal tubular element compresses and catches drive cable 33 while it is spinning, the electrical connections of transmission line 47 will be severed and the imaging catheter will no longer function. A competing requirement to keep the wall thickness as thin as possible exists in order to reduce the gap between the outside diameter of drive cable 33 and the inside diameter of outer distal tubular member 50, which will be further elaborated in the description of the current design shortcoming below.
The outer distal telescopic tubular member 50 is attached at its proximal end to anchor housing 29 which contains fluid seal 41. Fluid seal 41, applies pressure to inner proximal tubular member 26. For this reason, inner proximal tubular member 26 must have a smooth outer surface along its entire length in order to form a fluid seal. The distal end of the outer distal tubular member is bonded via glue 43, to strain relief 44 and proximal shaft 27, which is part of the catheter sheath 28. The inside diameter of the outer distal tubular member 50, is sized to accommodate the outside diameter of end stop 51 which must be larger than the outside diameter of the inner proximal tubular member 27. Therefore, the inside diameter of the outer distal tubular member 50 is larger then the outside diameter of the inner proximal tubular member 26. This creates a significant gap between the outside diameter of drive cable 33 compared to the inside diameter of the outer proximal tubular member 50.
This gap is a major deficiency of the current design. When the telescope is fully extended, the transducer is in its most proximal location within sheath 28. Since the lumen 49 is filled with saline, the distal housing 34 and drive cable 33, must displace this fluid as the telescope is retracted and the transducer 46 is advanced into the sheath 28. This creates a backward force on drive cable 33. Due to the gap between drive cable 33 and the outer proximal tubular member 50 and the flexible nature of drive cable 33, drive cable 33 is compressed into an “S” curve as shown in FIG. 2. This “S” curve pulls the location of transducer 46 inward, thereby scanning the incorrect region of the anatomy and often leads to the drive cable 33 folding back over onto itself. When the drive cable 33 folds back over onto itself, the electrical connections of transmission line 47 are severed and the imaging catheter is rendered inoperative. Approximately 1% of all IVUS catheters used are returned as defected units as a result of this failure mechanism.
Another short coming of existing telescope designs is that the telescope is not straight, which makes it difficult to extend and retract the telescope. This occurs because the telescope is made of polymers for cost reasons, and the inner proximal tubular member's 26 wall thickness is kept thin to keep the outside diameter of the outer telescope member as small as possible. The resultant thin wall polymer is then coiled into its packaging and during sterilization and normal shelf aging, the polymer takes a set in the coiled (non-straight) position.
Another short coming of existing telescopes is the fluid seal 41 and inner proximal tubular member 26 outer running surface. The fluid seal must prevent saline from escaping during catheter flushing operations. This fluid seal is subject to pressures up 150 PSI. Current telescopic sections are made from polymers that do not have a smooth running surface for the fluid seal to slide against during telescopic action. As a result, the pressure on the fluid seal is increased to insure the seal holds against the flushing pressure. This in turn increases the friction that must be overcome when the telescope is extended or retracted. As a result, the existing telescope design is difficult to extend and retract. Another failure mechanism occurs when the user forces the outer distal tubular member 50 downward onto the inner proximal tubular member 26 which is not straight and the inner tubular member is kinked. This results in either a failed electrical connection or a mechanical defect in the drive cable which manifests itself in a non-uniformed rotation of the transducer and the associated image artifact.
Another short coming of existing telescope design is the cost. The current design contains three separate tubular members which need to be individually bonded to form the telescope and this adds unnecessary cost to the assembly. The three components are the inner proximal tubular member 26, the outer distal tubular member 50 and the proximal shaft 27 of the catheter which is bonded to the distal end of the outer proximal tubular telescopic member.
A short coming of the existing drive cable 33 design is that it is flexible its entire length. This results in several shortcomings. The drive cable 33 can fail by folding back on itself as described above. The drive cable can fold back in the above S shape, while not failing, it pulls back the transducer 46 proximally so that it is pointing at a more proximal region of the artery then it should. This results in errors in length measurements on the system which is not aware of the fold hack condition. The flexible drive cable lacks torsional stiffness which can results in erratic rotational velocity of the imaging element. Erratic rotational velocity of the imaging element produces distortions in the image.